Loop antenna system



April 13, 1943.

W. VAN B. ROBERTS LOOP ANTENNA SYSTEM Filed June 29, 1940 INVENTCR rig/4N B. ROBERTS WAL A TTORNE Y Patented Apr. 13, 1943 UNITED STATE poor ANTENNA sYs'rEM Walter van B. Roberts,

of Delaware Princeton, N. J assignor to Radio Corporation of America, a corporation Application ml, 29, 1940, Serial No. 343,065 Claims. (01. 250 33),

The present invention relates; generally to signal pick-up devices and more particularly to'loop antennae; I

It is an object of the present invention to improve the efliciency and operating characteristics of loop antennae to the end that sufiicient voltages may be developed in small loops to operate normal receiving equipment.

It is desirable in some cases, .especially in high speed airplane applications, to reduce the size of the loop antenna since it has been proven that considerable increases in possible speeds of an airplane may be obtained by reducing the size of the antenna housing. In some instances, even a small reduction in the size of the loop antenna housing has been known to give an increase in speed of several miles'per hour'i'n a power dive.

In accordance with the present invention the size of a loop antenna is reduced by providing the loop with a core which is preferably composed of finely divided magnetic material such as powdered iron or magnetite.

Applicant is aware of the fact that it has been suggested previously to provide a loop antenna with a core of relatively high permeability compressed iron powder, the core being in the form of a relatively thin round disk. However, it has been found that such a core does not provide sufficient reduction in loop size to warrantthe added weight and expense of the core.

In accordance with the present invention, various loop and core arrangements are disclosed which provide an increased pick-up together with a relatively small inductance increase thereby providing sufiicient improvement in performance to justify the use of a magnetic core.

In order that the invention may be more readily understood the general case of a core which is ellipsoidal in shape will be considered. Such a core when placed in a uniform magnetic field, such as is the magnetic'field of a radio wave at a given instant and within a small volume of space, becomes uniformly magnetized throughout with an intensity of magnetization that is determined by the permeability of thematerial and by the de-magnetizing factor of the body. Since the'de-magnetizing factor for any ellipsoid has been worked out it may be readilydetermined, for any ellipsoid and any given magnetic material, how many times the total flux through a loop antenna'will be multiplied by the presence of the core. For the present it will sufllce to say that an elongated core gives a greater multiplier than a flattenedcore-and that in the. specialcase of a sphere of permeability 1,, the total number of lines of induction is multiplied by the quantity P'+2 It will readily be seen that. no matter how large the value of a may be, there will never be as much as three times as many lines of inductance threading a coil wound on a sphere as in the absence of the core. If, for example, the permeability of powdered iron is taken as 10, which is about as good as commercially available, the factor is 2 /2. On the other hand, even if the permeability has a value of only 4 which is about correct for magnetite, the factor is 2. Thus it is seen that in the case of the spherical core the lower losses and lighter weight of magnetite make it preferable to powdered iron since the magnetic multiplying action of the two materialsv is so nearly the same.

While a sphere of magnetite doubles the voltage per turn picked up in the loop, an elongated core shaped something like a football will in-.

crease the pick-up still more.

In the case of a core composed of magnetite with permeability of 4, the inductance of the loop is not increased byas much as 4, hence, to bring back the inductance to the value required for tuning by a given condenser, the turns will not need to be reduced to one-half. But the flux threading these turns is double in the case of a sphere. Hence, the totalvoltage generated in the loop is increased by the use of a spherical core even though theinductance is kept constant. Furthermore, since fewer turns are required the loop resistance may be greatly reduced and hence the sharpness of tuning and resonant rise of voltage increased.

A further improvement in loop performance-is obtained-by using a spherical or elongated core whose small diameter is sufficiently less than the diameter of the loop winding to provide a substantial clearance between the winding and the core. If the winding is concentrated so that the field of the loop is strongest near the winding, then such a core does not increase the inductance nearly so much as in the case where the core fits closely into the winding. On the other hand, there still remains a considerable increase in the total flux threading the loop. For example, if the cross-section of the core (assumed spherical) were A of the area inclosed by the loop, then the flux in the core itself would be 3 halves of they total flux that would thread the area of the loop occupied by the core inthe absence of the core,

and in addition to that there would also be some flux between the winding and the core, so that the total fiux would be something between 1 and 1% times the flux through a loop without a core. Since the latter figure is more nearly correct it will be seen that very little loss in total flux through the loop results from using a core enough smaller than the loop to have considerable clearance, while this clearance permits a considerable increase in the number of turns allowable for a given inductance, thus resulting in a further increase in total voltage pick-up.

The invention will be described more fully in conjunction with the accompanying drawing wherein, three of many possible forms which the invention may take are illustrated. In the drawing,

Fig. 1 is a diagrammatic representation of a cross-section through a loop provided with an ellipsoidal core which takes the special form of a sphere;

Fig. 2 is a diagrammatic representation of a section through a loop provided with an ellipsoidal core and showing the loop spaced from the core;

and,

Fig. 3 is a diagrammatic representation in cross-section of a stream-lined housing incorporating certain features of the invention.

Referring more particularly to Fig. 1 of the drawing, a loop antenna I is shown wound around a spherical core 2. The core while preferably constructed of magnetite may be composed of any other suitable magnetic material such as, for instance, powdered iron. The lines 3 represent the magnetic field in which the loop is placed. It will be noted that the iron core causes the lines of force to converge, thereby causing a greater number of flux lines to thread the turns of the loop. In the arrangement shown the loop is wound closely to the core. In such an arrangement the presence of core material close to the winding acts to increase the inductance of the coil. To overcome this objection an arrangement such as shown in Fig. 2 may be used.

In Fig. 2 the core 4 is made ellipsoidal in shape and is provided with a band of insulation material 8 of sufficient thickness to provide the desired spacing between the loop and core. For this purpose the loop is wound around band 8. to be understood that the core 4 may be made up of the same material as core 2 shown in Fig. 1.

In the event that it is desired to incorporate the features of the present invention in aircraft installations, the arrangement shown in Fig. 3 may be used. In this case the loop windings I may be embedded in the stream-lined housing which is preferably composed of insulation material. As noted from the drawing, thehousing 5 has sufiicient thickness to provide the proper spacing between the loop I and the core 1. Where a housing is employed as shown in Fig. 3, loose magnetic material such as crushed magnetite or black sand may be poured into the hollow portion of the housing through any suitable opening. As is common practice the housing 5 is desired to rotate the loop then a device snchas shown in Fig. 2 may be mounted'.within the housing 5 in place of the core I and suitable provision made for rotating. the core and associated loop.

In such a case care must be taken to make the dimensions of the loop and core such as to permit rotation within the housing. It is also possible to mount a device such as shown in Fig. 3 within a larger stream-linedhousing so as to permit rotation therein.

In the arrangements shown in Figs. 2 and 3 it is desirable to make the windings of the loop concentrated; In this way the field of the loop is made strongest near the winding at which location no core material is present thus preventing the core from appreciably increasing the inductance of the coil.

It should, of course, 'be understood that the core may be made into other shapes and that the term ellipsoidal as used herein is intended to indicate also cores which are approximately ellipsoidal in shape. Various formulae are given below which should be sufiicient for analyzing the performance of ellipsoidal cores. It will be understood, of course, that in actual practice cores of other shapes may be preferred and the ellipsoidal case has been discussed only because it is subject to exact analysis and serves as a rough guide to the performance of elongated cylindrical shapes.

Let K =susceptibility, ==permeability=l +41rK I-I=field before magnetic body is present =resultant field acting on the material =total flux density= H =intensity of magnetization=KH =de-magnetiza'tion factor and is defined by H=H'-NI From the above we easily deduce therefore B/H'=factor by which th total lines of induction are multiplied in an ellipsoid:

4-1r =approxunately O +1 if p. is large.

For a sphere N:

For long ellipsoids (prolate spheroids) Where e= 1a /c c=long radius. a=short radius. For short elliposids (oblate spheroids) Where e /1c /a c=short radius. a=long radius.

of the loop antenna is sufiiciently larger than the diameter of that portion of the core around which the loop antenna is wound to provide a gap between the loop antenna and the core which is large compared to the radial thickness of the loop antenna.

3. In an antenna system, a loop antenna, a housing for said loop antenna comprising a hollow body which is substantially ellipsoidal in shape, said loop antenna comprising a plurality of concentrated windings mounted near the outer surface of said housing, substantially coaxially with the major axis of the housing, the planes of each of said windings being substantially perpendicular to said major axis, said hollow body being substantially completely filled with loose comminuted magnetic material.

4. The system described in the next preceding rality of windings embedded in said shell near the outer surface thereof and so that the axis of the loop substantially coincides with the major axis of the ellipsoidal shell, said windings being concentrated in the vicinity of that portion of the shell which has the greatest radius of curvature, said shell being substantially completely filled with comminuted magnetic material.

WALTER VAN B. ROBERTS. 

